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Adds a GetKeyboardBrightnessPercent method to the PowerManagerClient
interface. This method was added in order to track keyboard energy
consumption over time so that chrome://power could approximate power
usage of other devices over time.

keyboard brightness change as you increase and decrease your keyboard
brightness.

BUG: crbug.com/851767
TEST: Call this method inside Chrome on a Chrome OS device and see the
Change-Id: I35323708da1e0bcc9b4549b85ad6e2d8ad215af5
Reviewed-on: https://chromium-review.googlesource.com/1120839Reviewed-by: Dan Erat <derat@chromium.org>
Commit-Queue: Nikhil Athreya <nathreya@google.com>
Cr-Commit-Position: refs/heads/master@{#572025}